Method for improving transparency of gadolinium molybdate single crystal

ABSTRACT

IN CASES WHERE A GADOLINIUM MOLYBDATE SINGLE CRYSTAL OF LOW TRANSPARENCY IS PRODUCED THIS CRYSTAL IS SEUBEQUENTLY SUBJECT TO HEAT TREATMENT UNDER SPECIFIED CONDITIONS OF TEMPERATURE AND TIME, THEREBY TO IMPROVED ITS TRANSPAR-   ENCY. IN ACCORDANCE WITH THE PRESENT METHOD, WHEN A CRYSTAL HAVING, FOR EXAMPLE, A LIGHT PERMEABILITY OF 20% AT A WAVELENGTH OF 6,328 A. WAS HEAT-TREATED AT 1,1000*C. FOR 5 HOURS, THE LIGHT PERMEABILITY BECAME 73% AND WAS IMPROVED BY 265%.

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mmm E I N r G EG E l R HEAT TREATMENT TEMPERATURE Lm -i I Us. or.423--263 United States Patent METHOD FOR IMPROVING TRANSPARENCY OFGADOLINIUM MOLYBDATE SINGLE CRYSTAL Seikichi Akiyama, Kokubunji, andI-Iirotsugu Kozuka and Koichi Megumi, Tokyo, Japan, assignors toHitachi,

Ltd., Tokyo, Japan Filed July 28, 1972, Ser. No. 276,175 Int. Cl. COlg57/00 19 Claims ABSTRACT OF THE DISCLOSURE In cases where a gadoliniummolybdate single crystal of low transparency is produced this crystal issubsequently subjected to heat treatment under specified conditions oftemperature and time, thereby to improve its transparency. In accordancewith the present method, when a crystal having, for example, a lightpermeability of 20% at a wavelength of 6,328 A. was heat-treated at1,l00 C. for hours, the light permeability became 73% and was improvedby 265%.

BACKGROUND OF THE INVENTION The present invention relates to a method ofsubjecting a gadolinium molybdate (Gd (MoO single crystal, particularlya crystal of low transparency, to heat treatment to thereby enhance thetransparency.

Since the gadolinium molybdate single crystal is a ferroelectric andferroelastic material and is light-permeable, it has excellent featuresas an optical material. The single crystal is usually produced by thefollowing procedure: Gadolinium oxide and molybdenum oxide are mixed ina mol ratio of 1:3. The mixture is fired at a temperature of 1,000 C. orso. Thereafter, the fired product is melted. With a seed crystal dippedin the melt, and then, with the seed kept rotating, the single crystalis grown by the pull method (generally called the Czochralski method).If the 'raw material is prepared so that the mol ratio betweengadolinium oxide and molybdenum oxide may become exactly 1:3 asmentioned above, the crystal can be produced. The mol ratio, however,fluctuates due to such causes as the precision of weighing atcompounding of the raw material, and a slight evaporation of molybdenumoxide in the process of producing the single crystal. Accordingly, itoften occurs that the composition ratio between the gadolinium oxide andmolybdenum oxide constituting the single crystal deviates from the molratio of 1:3. As the result, the transparency of the crystal isdegraded. In addition, the crystal at low transparency includes localinternal distortions. In case where such crystal is used for, e.g., anoptical shutter, a voltage (usually termed the threshold voltage)required for inverting the polarization of the crystal is 2-5 times ashigh as one in case of employing a crystal at high transparency.Moreover, absorption of light is greater. The crystal at lowtransparency is therefore unsuitable for practical uses.

As is well known, an optical crystal is required to be transparent. Inmany cases, however, neither natural crystals or artificial crystalshave sufficient transparencies. The opacity is generally considered tobe attributable to deposition of heterogeneous crystals, deficiency ofoxygen ions within the crystal, and the like. Also, in a gadoliniummolybdate single crystal to be treated by the method of the presentinvention, the deposition of such heterogeneous crystals as Gd O MoO andGd O 2MoO is known to cause low transparency.

The inventors have subjected such poorly transparent single crytal toheat treatment under various temperature conditions, and found that,when specific temperatures and periods of time are selected, thetransparency can be remarkably improved.

SUMMARY OF THE INVENTION An object of the present invention is toprovide a heat treatment method by which a gadolinium molybdate singlecrystal of light permeability less than 60% can be modified to a crystalhaving a light permeability at or above 60%.

The present invention has been accomplished on the basis of theheretofore described experiment, and is characterized in that agadolinium molybdate single crystal of low transparency is heat-treatedunder the conditions of temperature and time within the region A definedby a curve 1 in the accompanying figure as will be stated below. Thisfigure shows the conditions under which a light permeability of acrystal having permeability below 60% can be improved to at least 60%.The axis of ordinates in the figure represents the heat treatment time,and the axis of abscissas represents the heat treatment temperature. Thecurve 1 indicates the conditions under which the light transmissionfactor of 60% is attained, the region A on the right side of the curve 1(which also includes the curve 1) indicates the heat treatmentconditions of the present invention under which a transmissionpermeability of at least 60% is attained, and a region B on the leftside of the curve 1 is a part outside the scope of the presentinvention.

As is well known, a light permeability of at best approximately 60% hasbeen attained with the prior-art technique even in the crystal of themost excellent transparency. Moreover, the percentage of yield at whichsuch crystals having the light permeability of 60% were produced wasgenerally low, and most crystals did not reach 60% in the lightpermeability. It is accordingly very significant in the crystal industrythat a crystal below 60% in the light permeability is improved to one ofat least 60% in accordance with the method of the present invention. Howeffective the present invention is, will also be apparent from the factthat the light permeability required for optical materials is generallyaccepted as being 60 to or higher.

BRIEF DESCRIPTION OF THE DRAWING The figure in the accompanying drawingis a characteristic curve diagram showing the relations of temperatureand time which are the heat treatment conditions on gadolinium molybdatesingle crystals. In the figure, the region A divided by the curve 1(which includes the curve 1) gives the heat treatment conditions of thepresent invention, while the region B shows the condition of examplesused for comparison purposes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will bedescribed more concretely hereunder in connection with the preferredembodiments.

Examples The specimens of gadolinium molybdate single crystals shown inTable 1, which had been previously produced by the well-known pullmethod heretofore described and which exhibited various differenttransparencies, were heat-treated in accordance with the method of theinvention within air at temperatures and for the periods of time shown.Then, as set forth in the table, the resultant crystals were evaluated.The light permeabilities for all the specimens were at least 60%. Thelight permeabilities were measured using as a light source a He-Nelaserwhich had an output of 15 mw. and a wavelength of 6,328 A. In the table,results from heat treatment conditions outside the scope of the presentinvention are also shown as comparative examples. As apparent from thecomparative examples, outside the range of the present invention, thelight permeability could not be improved up to 60% for any specimen.

Cooling after the heat treatment in each case involved quick cooling ata cooling rate of 400 C./hr.

Effects of time and temperature on the light permeability of (Gd (MoOsingle crystals TABLE 1 Light per- Heat treatment Light permeabilityconditions meahihty before heat after heat treatment, temp., Time,treatment, percent 0. hr. percent Example No.:

It will be appreciated that the cooling rate is not restricted to 400C./hr. and that if a cooling rate of at least 200 C./hr. is employed,the crystals can be taken out of the heat treatment atmosphere withoutany deterioration of the transparencies improved by the heat treatment.In general the cooling rate is from 200 C. to 3000 C./hr. and preferably200 C. to l000 C./hr.

Furthermore, the heat treatment atmosphere is not restricted to air, butit may also be oxygen. In any case, an oxidizing atmosphere containingoxygen may be employed.

While the novel principles of the invention have been described, it willbe understood that various omissions, modifications and changes in theseprinciples may be made by one skilled in the art without departing fromthe spirit and scope of the invention.

What is claimed is:

1. A method for improving the transparency of a gadolinium molybdatesingle crystal, wherein a gadolinium molybdate single crystal of lowtransparency whose light permeability at a wavelength of 6,328 A. isbelow 60% is heat-treated under conditions of temperature and timewithin the region A that is enclosed by curve 1 shown in the figure andthereafter cooling at a rate of at least 200 C./hr., whereby the lightpermeability is enhanced to at least 60%.

2. The method according to claim 1, wherein the heat treatment isconducted within an oxidizing atmosphere containing oxygen gas.

3. The method according to claim 1, wherein the gadolinium molybdatesingle crystal is heated for 48 hours at a temperature between 970 to1100 C.

4. The method according to claim 1, wherein the gadolinium molybdatesingle crystal is heated at a temperature of about 970 C. for 20 to 30hours.

5. The method according to claim 1, wherein the gadolinium molybdatesingle crystal is heated for about 5 hours at about 1000 to 1150 C.

6. The method according to claim 1, wherein said gadolinium molybdatesingle crystal is heated at a temperature of about 1035 C. for 1 to 1.5hours.

7. The method according to claim 1 in which said gadolinium molybdatesingle crystal is heated to a temperature of about 1100 C. for about0.25 to 0.5 hours.

8. The method according to claim 1, wherein said gadolinium molybdatesingle crystal is heated to a temperature of about 1120" C. for about0.1 hours.

9. The method according to claim 1, wherein said gadolinium molybdatesingle crystal is heated to a temperature of about 1168 C. for about0.03 hours.

10. The method according to claim 1, wherein said gadolinium molybdatecrystal is heated to about 940 C. for about hours.

11. A method for improving the transparency of a gadolinium molybdatesingle crystal produced by the pull method, said gadolinium molybdatesingle crystal having a light permeability at a wavelength of 6,328 A.below 60% comprising heat treating said gadolinium molybdate singlecrystal under conditions of temperature and time within the region Athat is enclosed by curve 1 shown in the figure and thereafter coolingat a rate of at least 200 C./hr., whereby the light permeability isenhanced to at least 60% 12. A method according to claim 11, whereinheating of said gadolinium molybdate single crystal is accomplished at atemperature high enough and for a time long enough so that the lightpermeability at a wavelength of 6,328 A. is increased to 65% or higher.

13. A method according to claim 12, wherein heating is accomplished attemperatures high enough and for a period of time long enough so thatthe light permeability of said crystal is increased to at least about70%.

14. A method according to claim 11, wherein said crystal is cooled at arate of from about 200 C. to 3000 C. per hour after it is heat treated.

15. A method according to claim 14, wherein said crystal is cooled at arate of from about 200 C. to 1000 C. per hour.

16. A method according to claim 15, wherein said crystal is cooled at arate of about 400 C. per hour.

17. A method according to claim 14, wherein cooling is accomplished inan oxidizing atmosphere.

18. A method according to claim 11 wherein said heat treatment isaccomplished at a substantially constant temperature.

19. In a process for producing a plurality of gadolinium molybdatesingle crystals each having a transparency to light whose wavelength is6,328 A. on the order of 60% in which seed crystals of gadoliniummolybdate are immersed in a melt of gadolinium oxide and molybdate oxidein a molar ratio of 1:3 and thereafter each of said crystals is slowlywithdrawn from the melt by the pull method to produce a gadoliniummolybdate single crystal, the improvement for increasing the number ofcrystals having a transparency of at. least 60% comprising heat treatingeach of said gadolinium molybdate single crystals under conditions oftemperature and time within the region A that is enclosed by curve 1shown in the figure and thereafter cooling at a rate of at least 200C./hr.

References Cited UNITED STATES PATENTS 4/1969 Borchardt 423-263 3/ 1972Ballman et al 23-301 SP OTHER REFERENCES HERBERT T. CARTER, PrimaryExaminer US. Cl. X.R.

I' UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 83632 Dated September 17, 1974 Inventor(s) Seikichi Akiyama, HirotsuguKozuka and Koichi Megumi It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Title page, insert the following:

[30] Foreign Application Priority Date- July 28, 1971 Japan...56538/71Signedand sealed this 31st day of December 1 74.

(SEAL) AtteSt:

McCOY 2-1. GIBSONIJRI Arresting Officer c. E LARSHALL DANE- aCommissioner of Patents

